1. Field of the Invention
The present invention relates to photoelectric transducers with elements each of which performs photoelectric conversion (hereinafter, referred to as photoelectric conversion element) using photovoltaic effect of a P-N junction of a semiconductor, in particular, to improvements in performance of photoelectric transducers in which such elements are arranged into a two-dimensional matrix.
2. Description of the Related Art
Semiconductor solid-state image pickup devices are currently used in various fields of image pickup. Particularly in recent years, there is a growing tendency toward cost and size reduction and system integration, based on the more minute design rule. Solid-state image pickup devices can be applied to various fields, e.g., home electronic devices such as video recorders, information terminals, and simplified imagers.
Photoelectric conversion elements are the most principal components of such a solid-state image pickup device. The mainstream of such elements is in those using photovoltaic effect of a P-N junction of a semiconductor. They are roughly classified into CCD type and CMOS type in accordance with a difference in method for reading out signals obtained by photoelectric conversion. In CCD type, electric charges obtained by photoelectric conversion are transferred to the external through a charge coupled device (CCD) formed in a semiconductor chip. In CMOS type, electric charges obtained by photoelectric conversion are converted in a CMOS device into a voltage signal to output.
CCD type solid-state image pickup devices have been mainly used hitherto because of high sensitivity and little noise. Such CCD type solid-state image pickup devices, however, have drawbacks, e.g., that the power consumption is much, and a separate chip is required for signal processing circuits. Contrastingly, CMOS type solid-state image pickup devices have the advantages that the power consumption is little, and signal processing circuits indispensable to such a solid-state image pickup device can easily be integrated on the same chip. For this reason, cases of adopting CMOS type have increased. In order to make such advantages of CMOS type more efficient, it has been desired that the portion for performing photoelectric conversion, i.e., a photoelectric transducer, (hereinafter, referred to as photoelectric conversion portion), is manufactured by a minute process similar to that for peripheral signal processing circuits, so that the highly-minute peripheral signal processing circuits can easily be incorporated.
When the photoelectric conversion portion is manufactured by a minute process equivalent to that for various peripheral signal processing circuits comprising a solid-state image pickup device, however, the following problems arise because the impurity concentration in a well increases with making the process standard the more minute.
(a) As the impurity concentration in a well increases, the lateral location of a P-N junction shifts toward the surface area, and the depletion layer of the P-N junction becomes narrower. The efficiency of converting light energy into electric signals is very high in such a depletion layer. Thus the decrease in width of the depletion layer causes a reduction in sensitivity.
(b) The decrease in width of the depletion layer and the increase in impurity concentration in the well as described in (a), causes the strength of electric field at the P-N junction to increase. The increase in field strength causes an increase in tunnel current, which is a component of a P-N junction leakage current, through carrier trapping centers. This causes an increase in dark current level of the image pickup device.
In addition to those, a simple application of a minute process equivalent to that for peripheral circuits, to the photoelectric conversion portion, causes the following optical problem.
(c) In a general CMOS process, P- or N-type wells are almost uniformly provided in a semiconductor substrate. If such a process is applied to the photoelectric conversion portion, it may cause the phenomenon that some of carriers generated under a P-N junction by photoelectric conversion, flow into an adjacent pixel. The leakage of carriers to the adjacent pixel (cross talk) causes a reduction in resolution in case of monochrome image, besides, a mixture of colors in case of color image.
An example of improvement for such cross talk is disclosed in Japanese Patent Application No. 9-232555 (1997). It proposes that an impurity layer (N type) on the substrate surface side of a P-N junction which functions as a light-receiving diode, is so formed as to have the higher impurity concentration the nearer to the substrate surface. In this method, however, because the P-N junction of the light-receiving diode must be formed by, e.g., ion implantation process, the number of steps of manufacturing process increases. Besides, it is hard to be compatible with formation of a shallow junction indispensable to a minute CMOS process.
As described above, conventionally, even if a CMOS type solid-state image pickup device whose power consumption is little and which is easy to manufacture coordinately with indispensable peripheral signal processing circuits, is desired to be adopted, serious problems have not yet been solved that the above-described various reductions in performance are brought about due to an increase in impurity concentration in a well as the device is made minute.
It is an object of the present invention to provide high-performance and highly-reliable photoelectric transducers with improved sensitivity and reduced leakage current in which cross talk with an adjacent pixel is considerably reduced in case of application to CMOS type solid-state image pickup devices.
It is another object of the present invention to provide manufacturing methods of such photoelectric transducers.
A photoelectric transducer according to the present invention comprises a photoelectric conversion element which comprises a first conductivity type region and a second conductivity type region surrounding the first conductivity type region, to perform photoelectric conversion using photovoltaic effect of a P-N junction between the first and second conductivity type regions; and a second conductivity type well surrounding the photoelectric conversion element, and having a higher impurity concentration than the second conductivity type region.
On the well, a semiconductor element D1 such as a MOS transistor may suitably be formed.
It is suitable to make up a photoelectric conversion element array from such photoelectric conversion elements, and provide peripheral circuits comprising semiconductor elements D2 (e.g., CMOS transistors). In this case, since each semiconductor element D1 in each photoelectric conversion element and each semiconductor element D2 in each peripheral circuit have the same basic construction (gate electrode, source and drain, etc.), they can be formed almost in the same process.
The impurity concentration distribution in the second conductivity type region of the photoelectric conversion element is preferably set as follows. That is, the impurity concentration first increases gradually in the direction toward the interior, and then decreases gradually after a predetermined point of depth. Besides, the maximum peak value of the impurity concentration at the predetermined point of depth is lower than the maximum peak value of the impurity concentration of the well.
A manufacturing method of a photoelectric transducer according to the present invention comprises steps of: introducing second conductivity type impurities into a semiconductor substrate to form a second conductivity type region; introducing second conductivity type impurities into a region of the semiconductor substrate surrounding the second conductivity type region to form a well, such that the well has a higher impurity concentration than the second conductivity type region; and introducing first conductivity type impurities into a surface region of the second conductivity type region to form a first conductivity type region.
Also in this case, it is suitable to form a semiconductor element D1 on the well, to form semiconductor elements D2 for a peripheral circuit, or to control the impurity concentration distribution in the second conductivity type region of the photoelectric conversion element as described above.
According to the present invention, since the well having a high impurity concentration is provided to surround the photoelectric conversion element, the well functions as a potential barrier to carriers from a region (e.g., another photoelectric conversion element) adjacent to the photoelectric conversion element. Leakage of carriers between neighboring photoelectric conversion elements is suppressed thereby.
Besides, when the second conductivity type region of the photoelectric conversion element has an impurity concentration distribution in which the impurity concentration first increases gradually in the direction toward the interior, and then decreases gradually after a predetermined point of depth, the impurity concentration at the P-N junction can be set low. This brings about a wide depletion layer, and so makes it possible to improve sensitivity and reduce leakage current. Further in this case, by controlling the maximum peak value of the impurity concentration at the predetermined point of depth to be lower than the maximum peak value of the impurity concentration of the well, the above function of the well as a potential barrier can be insured more.
According to the present invention, since the well having a high impurity concentration is provided to surround the P-N junction type photoelectric conversion element, a considerable reduction in cross talk with an adjacent pixel can be realized when the present invention is applied to a CMOS type solid-state image pickup device. Besides, when the second conductivity type region of the photoelectric conversion element has an impurity concentration distribution in which the impurity concentration first increases gradually in the direction toward the interior, and then decreases gradually after a predetermined point of depth, and the maximum peak value of the impurity concentration at the predetermined point of depth is lower than the maximum peak value of the impurity concentration of the well, it becomes possible to improve sensitivity and reduce leakage current.